In many optical applications, modules are used to couple a light source to an optical fiber. The module may include one or more lenses that promote efficient coupling between the optical fiber and the light source. The light source may be formed of a succession of thin films on a semiconductor substrate, so as to define a Vertical Cavity Surface Emitting Laser (VCSEL). A VCSEL is a surface emitting laser. Another type of semiconductor laser used in telecommunications applications is referred to as an edge emitting laser, which may be further divided into subtypes that include Fabry Perot (FP) and Distributed Feedback (DFB) lasers.
Particularly within the field of data communications via optical signals, consistency with respect to certain optical properties is important in assuring proper operations. The power output (i.e., the light intensity) must remain above a predetermined level. The wavelength of the signal may also be significant. Various factors will cause changes in the optical properties. For example, a change in the temperature of the environment in which a laser diode resides will affect the laser emission wavelength. As another example, the bias current of the laser controls its output power. The aging of a laser diode also may affect its power output.
Techniques for monitoring and controlling properties of an output beam are known. FIG. 1 shows a prior art approach to monitoring and controlling an output beam of an edge emitting laser diode 10. The diode is shown as being mounted on a substrate 12. The laser diode emits an output beam 14 from a front facet 16 and emits a monitoring beam 18 from a rear facet 20. The output beam may be directed through optics 22, such as a lens which provides beam collimation. The beam is reflected by a 45 degree mirror 24 to an optical fiber 26 that has an optical axis perpendicular to the substrate 12. The 45 degree mirror may be used for applications in which the desired orientation of the beam from an edge emitting laser is to be the same as the conventional output beam orientation of a module that uses VCSELs.
Within the path of the monitoring beam 18 from the rear facet 20 of the edge emitting laser 10 is a detector 28 that generates a signal indicative of power. Because there is a known ratio between the power of the output beam 14 and the power of the monitoring beam, the signal from the detector may be used to identify the output power to the fiber 26. The electrical signal from the detector is directed to a controller 30 that is able to adjust the bias current of the laser 10. Thus, the signal from the detector provides feedback for maintaining the laser in a constant output power state. While not shown, the controller may also receive a signal from a temperature sensor. Then, the controller may adjust operations of a thermo-electric cooling (TEC) device or a heating device.
While the monitoring and controlling techniques described with reference to FIG. 1 operate well for their intended purpose, there are concerns. For example, the known ratio of the power of the two beams 14 and 18 is less reliable with respect to maintaining the output power to the fiber 26 if the output beam 14 is manipulated in a manner different than the monitoring beam 18. For example, in an Externally Modulated Laser (EML), the modulation which occurs for telecommunications or other applications does not affect the monitoring beam 18. Thus, a feedback signal from the detector 28 will not show all fluctuations of output power to the fiber.